First, try another browser to see if it's your browser. If you still get the same issue, check Windows Task Manager to see what program(s) is hogging up all the CPU. If you have a single core CPU with no hyperthreading available, you may not be able to take that route. If Windows Task Manager won't come up or shows no program hogging up the CPU, then you may have malware causing it - have you run a virus/spyware scan recently?